This invention relates to the field of reflective liquid crystal display (LCD) projection, and more particularly to an apparatus for enhancing contrast over a broad wavelength range in an LCD projector.
In conventional projection optical architectures, a polarizing beamsplitter (PBS) polarizes an incident light beam and directs that resulting beam to a reflective light valve. In a practical projection system incorporating a light beam having a finite cone angle, the skew rays (not parallel to the optical axis) produce a noticeable degradation in the contrast of the projection system.
One method for correcting such loss of contrast in a reflective liquid crystal display (LCD) is to use a single waveplate which is placed in the beam path and redirects (i. e. transforms) the polarization state of those skewed rays that are reflected from the reflective light valve such that they are effectively reflected by the PBS back toward the light source. In such waveplates, a phase shift is introduced in the light beam that is governed by the equation
"PHgr"=(2xcfx80/xcex)xcex94n(xcex)dxe2x80x83xe2x80x83[1]
where "PHgr" is the phase shift, xcex is the particular wavelength, xcex94n(xcex) represents a birefringence factor of a particular material structure, d is the thickness of the waveplate.
Although the use of such a waveplate significantly improves the system contrast, the single waveplate is designed to be a quarter-wave phase retarder for only one specific wavelength of the incident light beam. Since such quarter-wavelength waveplates are physically constructed to optimally transform only rays having that specific design wavelength, light rays having other wavelengths are not accurately transformed by this waveplate due to the phase mismatch. Again, a less than optimal contrast results. However, such wavelength dependency can be overcome through the use of achromatic waveplates, where phase retardance is independent of the wavelength of the light. See S. Pancharatnum, xe2x80x9cAchromatic combination of birefingent platesxe2x80x9d, Proceedings of Indian Academy of Sciences Vol XLI, No. 4, Sec A, 1955, pp 130-144, A.M. Title, xe2x80x9cImprovement of birefringent filters, 2: Achromatic Waveplatesxe2x80x9d Appl. Optics Vol 14, N1, pp 229-237, 1975, and GB 2331912 Patent xe2x80x9cOptical retardance devicesxe2x80x9d.
To overcome similar wavelength limitations in other applications, two or more waveplates have been combined to provide a fixed design phase delay and thus transformation of the polarization state over a broad wavelength range in a same apparatus. xe2x80x9cThe handbook of Optics Volume IIxe2x80x9d (SBN 0-07-047974-7 para 3:53) describes the construction of such a broadband waveplate-combination with each waveplate having a distinctive and dissimilar dispersive birefringent materials. By aligning or crossing the optical axes of the waveplates and by carefully selecting their dispersive properties, it is possible to achieve broadband waveplate performance.
A polarizing beam splitter (PBS) is combined with two or more waveplates, or retarders, to provide a projection optical system that has high contrast over a broadband wavelength spectrum (i.e. a neutral color dark state). Each individual retarder, which is designed to have a unique wavelength response, has its fast axis aligned either parallel or perpendicular to the fast axes of the other retarders in the combination. The resultant combinational retarder has an effective broadband wavelength response that is better than that of the individual retarders acting alone. In addition, the combination provides an effective retarder with a well-defined fast axis, which can be accurately aligned to a hypotenuse surface of the PBS.
By selecting the birefringence factor and thickness of the individual retarders, wavelength dependencies can be minimized or controlled, allowing the combinational retarder to approach true achromaticity. The combinational retarder working in conjunction with the PBS provides a significant improvement in contrast over a broad frequency range than has heretofore been possible.